This invention relates to an apparatus and a process for removing a laminated layer of non-metallic material from a metallic substrate with precision of depth and lateral dimension and without damage to the metallic substrate.
A fast and economic approach to the problem of stripping or skiving non-metallic layers from metallic substrates for various purposes has been long sought. For example, laminated flexible circuits consisting of the polyimide, known by the trademark Kapton, over FEP (fluorinated ethylene-propylene) Teflon adhesive over a copper substrate are widely utilized in electrical components by the electronics industries. A given circuit design may be produced from a blank laminated sample through the selective removal of portions of the non-metallic layers to expose bare copper. The precision required depends upon the particular application for the circuit and its electrical tolerances.
In the past, such selective removal of plastic laminate layers over metal substrates were produced in mechanical skiving operations. Such operations include a scribe or other cutting tool to physically penetrate the non-metallic layers and expose given amounts of the metallic substrate. Unfortunately, mechanical skiving has not been satisfactory where high precision and quality control are required. Unless extraordinary care is taken in a mechanical system, some portion of the metallic substrate can be is either scored or removed entirely along with the plastic cover layers. Such results are often variable and can lead to weakening or failure of the resultant circuit.
In addition to the difficulty in controlling the depth of cut in a mechanical skiving operation, the area, or lateral width of the cut, has also been a vexing problem. It has been extremely difficult to achieve a clean sharp edge in the plastic laminate at the cut boundary.
Starting with the premise that a metal substrate, such as copper, is more energy reflective in the infrared light region than a plastic laminate coating, a system has now been devised for virtually elminating the problems of precision depth and width skiving through the utilization of nonmechanical means for producing the skiving action.
The present invention utilizes a high intensity continuous wave CO.sub.2 laser having a masked mirror-focused beam which is caused to nutate and expose selected areas of a masked sample mounted upon a translatable bed. The sample is exposed, for precisely calculated time intervals, to a linearly circularly sweeping high intensity beam which removes a non-metallic laminate over-layer through a combination of vaporization and explosion processes.
Numerous attempts have been made in the past for utilizing laser systems in drilling and cutting operations. For example, U.S. Pat. No. 3,832,948 to Barker discloses the utilization of a laser beam for selectively removing portions of a top layer from a laminated substrate. It should be noted, however, that the patented system is adopted for removing a thin metallic film over a plastic substrate and it relies upon the ability of the thin metal film to absorb visible or near visible laser light to create heat which breaks down the metallic structure. Barker discloses the use of a very lower power laser with which the problem of controlling lateral spread in the hole or cut produced is virtually non-existant. Also, in the Barker system one must alter the laser to utilize wave lengths in the infrared zone, which are absorbed by plastics, when removal of the plastic substrate is desired.
In contradistinction to the Barker system, the present invention comprises a one-step process for removing a plastic film over a metallic substrate with virtually no removal of the metallic substrate. Utilizing a high intensity laser, it operates through the absorption of infrared light by the plastic and through reflection of such light by the metal substrate to provide a combination vaporization and explosion removal of the plastic material in a precisely controlled area. The specialized purpose of this invention is to free selective metallic regions which are encapsulated in plastic for the purpose of making clean electrical connections without affecting surrounding plastic or metal.
An example of a prior art hole "punching" system, using a laser, is taught by U.S. Pat. No. 3,742,182 to Saunders. The Saunders device is capable of scanning across open areas of a mask to affect underlying surface material. Saunders discusses a method for scanning a laser beam in a circular path, but the disclosed technique is apparently adapted for boring or punching a hole through a thin piece of nonmetallic material. In Saunders, a binder material in a ceramic particulate base is vaporized and physically blown away from a hole by means of a jet of compressed air. The patentee states that his concern is producing holes in "sheets of non-homogenous material having finely divided particles held together by a binder".
Saunder's type systems would not be useful in the context of the present invention wherein the punching of holes through a non-homogenous material is not involved. A very precise system must be utilized when it is intended to remove select portions of a homogenous plastic coating over a metal substrate with no injury to the substrate and without external mechanical devices, such as pumps, to remove materials affected by the laser beam.
Unlike in the prior art system, the present invention utilizes a mask to assist in controlling the size, location, and shape of material being removed from a given area. Through the circular scanning of a laser beam within the confines of a given mask opening, the precise time and area of exposure of the beam to the plastic may be controlled. If one were to look microscopically at an area to be skived, the time of exposure of any given particle of plastic to the laser beam would be seen to be the same as that of any other particle. This is impossible with prior art systems.
In the present invention, a high powered laser, having a beam intense enough to vaporize and explode materials to be removed, is used. Only the vertical components of a nutating laser beam are used, with an exposure time interval brief enough that material which is not to be removed is "unaware" that the process is taking place.
Other prior art patents which are directed to certain per se aspects of the present invention are U.S. Pat. Nos. 3,965,327 and 3,786,224 to Ehlscheid et al. and Heywang et al., respectively.